Silicon dendritic-web crystals are long, thin ribbons of single crystalline material of high structural quality which can be grown in the (111) orientation. The current impetus for developing silicon dendritic-web is its application to the production of low-cost, highly efficient solar cells for direct conversion of sunlight to electrical energy. The thin ribbon form of the crystal requires little additional processing prior to device fabrication, in contrast to wafer substrates from the more traditional Czochralski crystal which must be sliced, lapped and polished prior to use, a costly process even though large volume economies are practiced. Additionally, the rectangular shape of the silicon ribbon leads to efficient packing of individual cells into large modules and arrays of solar cells.
FIG. 2 shows a typical system used for dendritic-web crystal growth. As shown, a susceptor 28 having a susceptor cavity 29 contains a crucible 30 containing molten polycrystalline silicon 31. A susceptor lid 15 is positioned over the crucible/susceptor system. The susceptor lid 15 contains a slot 16 through which dendritic web crystals 32 can be pulled. As shown, the dendritic web crystal 32 is bounded by a bounding dendrite 10 which is immersed in the molten polycrystalline silicon 31. Also shown in FIG. 2 are radiation shields 20 spaced above the lid 15. As shown, the radiation shields 20 also contain slots 16 through which the dendritic web crystal 32 may be pulled.
For technical and economic reasons it is highly desirable that these ribbons be grown at a predetermined, fixed width which matches the requirements for ultimate fabrication into semiconductor devices, such as solar cells. In most of the growth configurations which have been used until now, the growing dendritic web continuously widens until the crystal deforms under the influence of thermal stresses. The physical mechanisms involved in the widening of the dendritic web can be readily understood from the schematic diagram in FIG. 1. The bounding dendrites 20 propogate in very nearly a [211] crystallographic direction as the result of the crystallographic symmetry considerations of the reentrant corner twin plane mechanism. This growth symmetry of the bounding dendrites is perturbed, however, by the lateral temperature gradients generated by the lateral heat loss as indicated in FIG. 1. Heat loss around the dendrites 10 is asymmetrical, being greater at the outside edges 11 and less at the dendrite faces 23 and inside edges 12. As a result of these gradients, the dendrites grow slightly more on their outside edges 11 than on the inside edges 12 with the result that the web crystal widens as it grows longer. Also as shown in FIG. 1 is an intrinsic temperature profile at the surface of the melt, a profile resulting from heat loss through the slots and holes in the lid which covers the susceptor as in the typical growth system shown in FIG. 2. Theoretically, growth at a constant width will result if the temperature gradient resulting from the lateral heat loss is balanced by a temperature gradient resulting from the intrinsic melt profile. In principal, it should be possible to control the width of a growing dendritic web crystal by changing the system temperature so that the crystal grows at the appropriate position in its "thermal trough." Indeed, it has been possible to control the width of the dendritic web by such a technique. However, in practice it has been found that the required balance is so critical that any slight temperature fluctuation will cause the dendrite to stop growing and the crystal pulls out of the melt.